1. Field of the Invention
The present invention relates to a semiconductor laser, and more specifically to a semiconductor laser which can be used as a light source in data processing instruments and light communication instruments.
2. Description of Related Art
Semiconductor lasers are extremely small in size and suitable to mass production. At present, therefore, the semiconductor lasers have been used as a light source for various light-electric instruments including data processing instruments and optical communication instruments. Among the semiconductor lasers, an AlGaInP semiconductor laser having an oscillation wavelength corresponding to a high visual sensitivity has been effectively used for miniaturizing a laser beam pointer. A main characteristics required in practical semiconductor lasers installed in general purpose devices such as the laser beam pointer, is that an oscillation threshold current is low and a stable operation is possible at temperatures ranging from 40.degree. C. to 60.degree. C. Recently, Kobayashi et al reported the AlGaInP semiconductor laser shows the oscillation threshold current as low as 40mA (SPIE International conference Letters, Vol. 898, Page 84, 1988).
In the case that the semiconductor laser is installed in the laser beam pointer, the volume of a battery cell used for driving the semiconductor laser occupies almost the volume of the laser beam pointer. Therefore, in order to further miniaturize the laser beam pointer, it is necessary to reduce the oscillation threshold current of the semiconductor laser, thereby to decrease the power consumption.
Furthermore, Katsuyama et al reported in Electronics Letters that a semiconductor laser using a strained quantum well active layer formed of Ga.sub.0.43 In.sub.0.57 P wells shows a low oscillation threshold current (Vol. 26, Page 1376, 1990). In addition, Ijichi et al reported that a semiconductor laser using the strained quantum well active layer formed of In.sub.0.22 Ga.sub.0.78. As shows a low oscillation threshold current (12th Semiconductor Laser International Conference Digest, Page 44, 1990). It has been considered that, in these semiconductor lasers, since an effective mass of valence band holes in the quantum well active layer subjected to an in-plane compressive strain becomes small, an oscillation threshold current is decreased.
The conventional semiconductor laser using the strained quantum well active layer has a low oscillation threshold current, but the high-temperature operation characteristics is not necessarily good. The reason for this is that the density of carriers injected into the active layer is high, and confinement of the carriers is not sufficient. In order to improve the high-temperature operation characteristics, it is necessary to further decrease the oscillation threshold current.